1. Field of the Invention
The present invention generally relates to carrier recovery. More specifically, the present invention relates to the recovery of carrier offsets using a timing recovery loop.
2. Background Art
Communication systems allow for communication over large distances by using sophisticated transponders in a satellite. Transponders may sometimes operate on an interplanetary scale. Transponders receive incoming communications over a band of frequencies, the uplink, and retransmit the communications over another band of frequencies, the downlink, at the same time. For example, transponders most frequently use the C band with an uplink from 5,925 to 6,425 MHz and a down link from 3,700 to 4,200 MHz. The uplink originates from a point on surface of the Earth, usually a ground station, to the transponder. The transponder sends the downlink to a point or region on the surface of the Earth, usually to one or more ground stations or receivers.
Communications in a digital communication system requires convergence of several signal processing algorithms before the receiver, usually located in the downlink, can output meaningful data. One such signal processing algorithm uses a timing recovery process to obtain symbol synchronization. Symbol synchronization involves determining the sample frequency and sample phase of the received symbol. The determination of the sample frequency requires an estimate of the symbol period to sample at the correct rate. The sample phase involves determining the correct time within a symbol period to take a sample.
After obtaining symbol synchronization, another signal processing algorithm uses a carrier recovery process to remove unknown frequency offsets from the received communication signal. Ideally, the frequency of an oscillator in the digital receiver system will match the frequency of an oscillator used at the transmitter. In practice, their frequencies differ. For example, the motion of the satellite or any variation in the oscillator of the receiver can cause the frequency difference between the transmitter and receiver oscillators. When the frequency of the transmitter oscillator differs from the frequency of the receiver oscillator, the process of down-conversion results in an unknown offset in the frequency content of the received communication signal relative to the transmitted communication signal. The receiver may use a carrier recovery loop to remove these unknown frequency offsets from the symbol content of the received communication signal.
Unfortunately, the timing recovery process and the carrier recovery process are not always independent. In fact, a large carrier offset may prohibit the timing recovery process from obtaining symbol synchronization. As a result, the data of the receiver may be corrupt. To correct for large carrier offsets, some conventional digital receiver systems achieve a coarse carrier offset acquisition by blindly stepping through carrier offsets and determining the convergence of the complete receiver. The complete receiver may consist of a timing recovery loop for the timing recovery process, a carrier recovery loop for the carrier recovery process, and additional forward error code (FEC) modules. Blindly stepping through carrier offsets and determining the convergence of the complete receiver may require a significant amount of time. Other conventional techniques for detecting and correcting for large carrier offsets require the use of non-decision directed digital PLLs for carrier recovery that perform poorly at low signal to noise ratios, and require the use of known data symbols. Further, some conventional digital receiver systems perform high level “tests” on a set of carrier frequency offsets that are complex and use the complete receiver and forward error code (FEC) acquisition as the “pass/fail” metric.
Therefore, what is needed is a digital receiver system to quickly and efficiently correct for large carrier offsets.